Among methods of electroplating non-conductive plastic moldings to form a decorative coating, a widely employed method comprises, in succession, degreasing, etching, optionally neutralizing and pre-dipping, then applying a catalyst for electroless plating using a colloidal solution containing a tin compound and a palladium compound, and optionally activating (accelerator treatment), followed by electroless plating and electroplating.
In such plating method, electroless copper plating solutions are frequently used which contain a reducing agent having a high reducing power such as paraformaldehyde. When this electroless copper plating solution is used, copper is deposited in the initial plating stage on a highly catalytic palladium portion of tin/palladium colloid coating applied as the catalyst. However, copper is continuously deposited by reduction due to the reducing action of the reducing agent with high reducing power and thus a copper layer is formed not only on the palladium portion but also in the transverse direction. As a result, copper is also deposited on an inherently non-catalytic tin portion so that a so-called bridge deposit is formed which is likely to provide a spongy coating. When electroplating is conducted on the electrolessly plated coating having the bridge deposit, pit-like fine agglomerates (called stardust) are partially deposited in a great number, thereby tending to render the coating irregular. The plated coating thus formed on the plastic substrate often shows an inferior appearance compared with a plated coating on a metallic substrate.
To prevent the creation of such inferior appearance, attempts have been made to brush the surface of the substrate electrolessly plated with copper before electroplating. However, the attempt is disadvantageous in involving a cumbersome step.
Further, the electroless copper plating solution contains, as a reducing agent, paraformaldehyde which is highly toxic and presumably carcinogenic. A further problem is that considerable labor is entailed in removing the metal ions in the disposal of waste water because a completing agent having a high complexing power such as EDTA has been used to solubilize the copper ions.
When electroless nickel plating is conducted in place of electroless copper plating, stardust scarcely develops because of a lesser degree of bridge deposits so that the resulting electroplated coating is esthetically superior in appearance to the electroplated coating formed on the electrolessly plated copper coating. However, electroless nickel plating disadvantageously requires more catalyst than electroless copper plating, thereby incurring higher costs. Further, when copper electroplating is conducted after electroless nickel plating, the nickel coating is eroded by the galvanic potential difference due to lower potential of the nickel coating than the copper coating, thereby tending to induce coat peeling and resulting in blisters.
A method is known which comprises immersing a substrate in a colloidal solution containing a precious metal to deposit a colloid coating of precious metal, and directly electroplating the coated substrate without conducting electroless plating (Japanese Unexamined Patent Publication No.267393/1991). However, this method is defective as follows. Since the coating has a low electrical conductivity and electroplating proceeds at a low deposition rate, the feeding points of electrical power require a large area when plastic moldings of great area are electroplated. Furthermore, a substantial time is taken to electroplate the entire substrate surface of such plastic moldings and it is difficult to form a uniform coating over the entire substrate surface. Moreover, the procedure is performed under narrowly limited conditions, and the control of treating solutions and the working conditions are very cumbersome.